This invention relates to a secondary element having a positive electrode, a negative electrode, and a non-aqueous electrolyte, in which the material of the respective electrodes forms an open grid or skeleton structure. This makes the electrode structure capable of acting as a "host" for a recipient compound, for alternately accepting and releasing electrochemically active cations during charging and discharging.
An important impetus for this invention was the recent, great increase in demand for batteries having high energy density and low weight, such as had already been achieved particularly with the lithium systems, but which are also rechargeable. This requires, among other things, that the electrodes be chemically stable in contact with the electrolyte.
Lithium electrodes do not meet these requirements when in use over extended periods of time, even in organic electrolytes with an aprotic solvent, because their cycling stability is well known to be sharply limited. This drawback can be overcome by alloying lithium with an alkaline earth metal or earth metal, preferably aluminum. By so doing, the reduced energy content of the alloyed electrode is offset by the benefit of better rechargeability and higher mechanical strength.
Another recent approach to improve the reversibility of lithium electrodes involves intercalation compounds. With such compounds, the cell's electrode incorporates a material of predetermined structure which form an appropriate "host" or recipient grid for electrochemically active species of ions that are present in the electrolyte. These ions, in this instance Li+, are either stored or released depending upon the polarity of an externally applied potential. During discharge, the electromotive force which is produced, and which manifests itself in the tendency to again reverse the forced intercalation or deintercalation, is used for current production.
From the outset, carbon of predetermined structural characteristics has proven suitable as the electrode material dopable with Li+ ions for both electrode polarities. For example, the electrodes of the electrochemical battery disclosed in German patent publication (DE-OS) 3,231,243 involve such products formed from active carbon. According to European patent application (EP-A) 165,047, the carbon material can be a pseudographite of predetermined crystalline size and with a lattice expanded in the direction of the c-axis which is obtained by pyrolysis from aromatic hydrocarbons. In a secondary battery disclosed in European patent application (EP-A) 201,038, which has as an electrolyte a solution of a lithium salt in a non-aqueous solvent, such a pseudographite forms the negative electrode, and a metal chalcogenide which is also capable of being doped forms the positive electrode.
The intercalation capability of metal chalcogenides, e.g. of WO.sub.3 or TiS.sub.2, as well as of certain synthetic mixed oxides disclosed in U.S. Pat. No. 4,668,595, is based upon their well defined lattice layers. The same printed publication also discloses chemically stable n-type material in the form of a carbon product, which is obtained from high molecular weight components of crude oil by a controlled coking process. In such case, for the purpose of incorporating metal cations, particularly Li+, a certain irregularity or lack of organization in the fine structure of the product is desired.
Finally, U.S. Pat. No. 4,507,371 discloses that, in rechargeable cells, host oxides or sulfides having crystalline chemistry of the spinel type can be used as either cathode or anode materials, and even as electrolyte if no electron conductivity is present. These spinel structures have high inherent stability, or can be stabilized if needed by the incorporation of certain cations such as Mg.sup.2+, Zn.sup.2+, Cd.sup.2+.
In particular, German patent publication (DE-OS) 3,736,366 discloses that pure lithium-manganese spinel, in which lattice the Li ions have high mobility, can be produced through the transformation of manganese dioxide (MnO.sub.2) with lithium salts at only moderately high temperatures of 300.degree. C. to 400.degree. C. This is what makes such spinels suitable as the active cathode material, particularly for rechargeable galvanic elements. In the charged state, such spinels have the formula LiMn.sub.2 O.sub.4, and in the discharged state, the formula LiMnO.sub.2. Through acid treatment, the lithium manganese spinel can be transformed into a lithium-poor compound without modification of its spinel structure, and with only a minor contraction of the cubic lattice.
In all previously known secondary elements with cathodes including a material with spinel structure, the associated anode is either a lithium alloy, an electrically conductive polymer doped with lithium ions such as polyacetylene or polyparaphenylene, or an intermediate layer compound of the TiS.sub.2 -type, which has lithium ions in the intermediate layer spaces, or else it is formed of a spinel type, like the cathode.